Each dungeon key is charged twice per day. (As of 00:00 UTC)

But, Slime Legion dungeon is recharged once a day.

After using all the dungeon keys, you can enter after watching the advertisement. (up to twice per day).

If you fail to clear the dungeon after watching the advertisement, it will be changed to a dungeon key and you can enter the dungeon without watching additional 

advertisements.

However, the dungeon key will be initialized at the same time as the next day's reset.